System and method for second order multi-layer traffic grooming for optical network optimization

ABSTRACT

A method includes forming a set of direct connections (including a high-speed connection and a low-speed connection) between an origination central office (OCO) and a destination central office (DCO). The method includes forming a spoke connection between the OCO and a hub node and forming a connection between the hub node and the DCO. The spoke connection is formed to carry first residual demand traffic from the OCO to the hub node. The connection is formed to carry the first residual demand traffic and second residual demand traffic (received at the hub node from other OCOs) from the hub node to the DCO. The method includes determining a first estimated cost (of forming the set of direct connections) and a second estimated cost (of forming the spoke connection and the connection). The method includes determining whether the first estimated cost exceeds the second estimated cost.

CLAIM OF PRIORITY

This application is a continuation of and claims priority from U.S. patent application Ser. No. 12/500,221, filed on Jul. 9, 2009, and entitled, “SYSTEM AND METHOD FOR SECOND ORDER MULTILAYER TRAFFIC GROOMING FOR OPTICAL NETWORK OPTIMIZATION,” which is a continuation of and claims priority from, U.S. patent application Ser. No. 10/884,381, filed on Jul. 2, 2004, now issued as U.S. Pat. No. 7,577,360, and entitled, “SYSTEM AND METHOD FOR SECOND ORDER MULTILAYER TRAFFIC GROOMING FOR OPTICAL NETWORK OPTIMIZATION,” the contents of each of which are expressly incorporated herein by reference in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to the design of multi-level transport networks.

BACKGROUND

The standards set forth by the Synchronous Optical Network (SONET) define optical carrier (OC) levels and the electrically equivalent synchronous transport signals (STS) for the fiber-optic based transmission hierarchy. For example, an OC-1 is equivalent to an STS-1 and provides a data transmission rate of 51.84 Mega bits per second (Mbps). Higher line rates are integer multiples of the base rate of 51.84 Mbps. In other words, an OC-3, and the corresponding STS-3, has a data transmission rate equal to 3 times 51.84 Mbps or 155.52 Mbps. Under the SONET standard, OC-3, OC-12, OC-48, and OC-192 are the most widely supported fiber-optic rates. However, other rates exist, e.g., OC-9, OC-18, OC-24, and OC-36.

As such, in a telecommunication network, there can be numerous types of connections that are established to handle signal traffic at the different transmission rates. These connections can include any combination of OC-1 connections, OC-3 connections, OC-12 connections, OC-48 connections, and/or OC-192 connections. In order to efficiently handle the signal traffic, it is often necessary to groom the signal traffic traveling over the network. Grooming involves rearranging and repacking low-speed demand, e.g., DS-1 demand, into higher speed connections, e.g., STS-1 connections, to obtain high utilization or fill ratios. Grooming allows demand from various destinations to be combined over a single transport connection.

In SONET ring based networks there are two ways to groom demand: centralized ring bandwidth management and distributed ring bandwidth management. In centralized ring bandwidth management, also known as “full hubbing,” all DS-1 demand originating from a node is packed into one or more STS-1 demands and transported to a central hub node. At the central hub node, the incoming STS-1 demands are dropped from the ring and connected to a wideband digital cross-connect system (WDCS). Within the WDCS, the individual DS-1 demands are cross-connected to groomed outgoing STS-1 demands, which are added back to the ring. Then, a number of co-destined demands can be transported to their common final destination.

An alternative to hubbing is distributed bandwidth management. In distributed bandwidth management, DS-1 demands are routed directly within the ring over a shared or collector STS-1. A collector STS-1 is an STS-1 time slot that is accessible by more than one pair of nodes. In order to access a collector STS-1, however, an add drop multiplexer (ADM) must be capable of time slot assignment (TSA) at the VT-1.5 virtual tributary level.

In a multi-level network, i.e., a network including an electrical level and one or more optical levels, the concept of traffic grooming, can be used when there is a significant volume of low-speed traffic between two nodes, e.g., central offices (COs). The demand in the multi-level network can be groomed to share the resources in the electrical and optical levels, thereby reducing the cost of the network. However, if each level is analyzed independently, it is possible that saving costs in one level, e.g., an optical level, can have a negative impact on the costs associated with the other level, e.g., the electrical level, and vice-versa. On top of this, if a SONET network is already installed in the field, it is necessary to make use of the SONET network in order to provide a smooth transition from the installed SONET network to a wavelength division multiplexing (WDM) based full optical network.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is pointed out with particularity in the appended claims. However, other features are described in the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1 shows the main building blocks for application of a single layer grooming scheme used in a SONET transport network optimization process including computer systems that pre-process network data and perform network analysis and planning;

FIG. 2 is a diagram representative of three distinct levels of a transport network that can be a candidate for implementation of multi-layer grooming approach;

FIG. 3 is a diagram of an embodiment of a second order single layer grooming model;

FIG. 4 is a diagram of an embodiment of a second order multi-layer grooming model;

FIG. 5 is a flow chart to illustrate general operating logic of an embodiment of a grooming tool; and

FIG. 6 and FIG. 7 depict a flow chart to illustrate detailed operating logic of an embodiment of a multi-layer grooming tool.

DETAILED DESCRIPTION

A method of planning a network is disclosed. The method includes estimating, at a processor, a first cost of forming a direct connection between an origination central office and a destination central office. The method also includes estimating, at the processor, a second cost of forming a spoke connection between the origination central office and a hub node. The spoke connection is to carry residual data traffic to be groomed with other traffic destined for the destination central office. The method also includes determining whether to implement the direct connection based on whether the estimated first cost exceeds the estimated second cost.

Another particular embodiment is a method that includes estimating, at a processor, a first cost of implementing a direct connection between an origination central office and a destination central office. The method also includes estimating, at the processor, a second cost of implementing a spoke connection between the origination central office and a hub node. The spoke connection is to carry residual data traffic to be groomed with other traffic destined for the destination central office. The method further includes adding to the estimated second cost an incremental cost of a light path to carry traffic from the hub node to the destination central office to yield an estimated third cost. The method also includes comparing the estimated first cost and the estimated third cost and selecting an implementation having a lower cost.

Another particular embodiment is a computer program embedded in a computer-readable storage medium. The computer program includes instructions to determine a first cost of a direct connection between an origination central office and a destination central office. The computer program also includes instructions to determine a second cost of a spoke connection between the origination central office and a hub node. The spoke connection is to carry residual data traffic to be groomed with other traffic destined for the destination central office. The computer program also includes instructions to select either the first direct connection or the spoke connection based on a comparison of the first and second costs.

Referring now to FIG. 1, a computer system for executing network planning software is shown and is generally designated 2. As shown, the computer system 2 includes a microprocessor 4 that has a memory 6, e.g., a hard disk drive. FIG. 1 shows that the computer system 2 further includes a display device 8, e.g., a monitor, upon which a user can view network plans developed using the network planning software described in detail below. Additionally, the computer system 2 can include an output device 10, e.g., a printer.

FIG. 1 further illustrates functionality of a non-limiting, exemplary embodiment of network planning software (S-TOP), designated 12, that can reside within the microprocessor 4, e.g., within the memory 6. In general, the network planning software 12 includes a data gathering module 14, a data manipulation module 16, a network design module 18, and a validation and reporting module 20. As shown, the network planning software 12 can include a grooming tool 22, e.g., within the data manipulation module 16, and a network planning tool 24, e.g., within the network design module 18. As described in detail below, the grooming tool 22 can be used to groom, or otherwise transform, network demand data 26 that is input to the grooming tool 22, e.g., via the data gathering module 14, in order to produce interim demand files 28. The interim demand files 28, in turn, can be input to the network planning tool 24 where they are used during the design of a network plan. As further shown in FIG. 1, one or more grooming parameters 30 can be input to the grooming tool 22 and can be used to determine how the demand data 26 is groomed by the grooming tool 22. A description of the grooming tool logic is provided below.

Referring still to FIG. 1, network architecture data 32, such as fiber and system topology data, can be input to the network planning tool 24 in addition to the interim demand files 28. Also, as the network planning tool 24 is used to design a network plan, a user can make a ring topology selection 34 based on the design choices presented by the network planning tool 24. As further shown in FIG. 1, during the design of a network plan by the network planning tool 24, a ring design cycle 36 and a robustness testing cycle 38 can be performed. FIG. 1 also shows that the network planning software 12 can include plural design metrics 40 that can be used by the network planning tool 24 during the ring design cycle 36. Once the network planning tool 24 has satisfactorily completed the ring design cycle 36 and the robustness testing cycle 38, network engineering reports 42 can be created by the validation and reporting module 20 and further, can be output by the network planning tool 24, e.g., via the output device 10. The network engineering reports 42 can include STS-1 & OC routing plans.

FIG. 2 shows an embodiment of a multi-level transport network, generally designated 50, that can be groomed using the grooming tool 22 described above in connection with FIG. 1. As illustrated in FIG. 2, the multi-level transport network 50 can include an electrical level 52, e.g., a SONET level; a first optical level 54, e.g., an optical add/drop multiplexer (OADM) level; and a second optical level 56, e.g., a wavelength division multiplexing (WDM) level 56. FIG. 2 depicts that the electrical level 52 can include plural electrical nodes 58 interconnected to each other. The electrical nodes 58 can include plural SONET add/drop multiplexer (ADM) nodes, plural digital cross-connect system (DCS) nodes, a combination of SONET ADM nodes and DCS nodes, etc. The first optical level 54 can include plural optical nodes 60 that are interconnected to each other. In one embodiment, the optical nodes 60 can include OADM nodes, optical edge device nodes, a combination of OADM nodes and optical edge device nodes, etc. As shown in FIG. 2, the second optical level 56 can further include plural optical nodes 62. The optical nodes 62 of the second optical level 56 can include plural optical cross-connect (OXC) nodes or plural multi-service transport switch (MSTS) nodes that are interconnected to each other.

It can be appreciated that the interconnection between the electrical level 52 and the optical levels 54, 56 can be facilitated by specified nodes, i.e., hub nodes or gate nodes. Further, the gate nodes can have different optical and electrical functionalities. For example, for low-speed traffic originating in the electrical level 52 to be routed through the optical levels 54, 56, electrical-optical (EO) conversions and optical-electrical (OE) conversions are necessary at designated gate nodes. Between optical nodes 60, 62, high-speed traffic can be transmitted only through optical levels 54, 56. However, the low-speed traffic between electrical nodes 58 can be transmitted either through the electrical level 52 or through the optical levels 54, 56. The low-speed traffic demands can be bundled together into higher level routes or even light paths, e.g., an OC-48 (2.5 Gbps) connection or an OC-192 (10 Gbps) connection, taking a common route in an optical levels 54, 56 thereby reducing costs.

As previously stated, optical-electrical-optical (OEO) conversions are needed in each designated gate node in order to transfer signals between the electrical level 52 and the optical levels 54, 56. In the present disclosure, demand originating in the electrical level 52 can jump to the optical levels 54, 56 and back, i.e., undergo an OEO conversion, a limited number of times. The intent is to avoid large delays during transmission caused by the OEO conversions. Additionally, due to the risk of problems associated with the failure of an OEO converter, a large number of OEO conversions can have a strong impact on network survivability and performance. Each level can provide the protection/restoration mechanism for the connections that exist in that particular level—without exchanging information with other levels.

Typically, when low-speed traffic is groomed into a light path and routed through the optical levels, the intermediate optical nodes cannot access the low-speed traffic. If part of the low-speed traffic has to be dropped at an intermediate node, the entire light path is converted to an electrical signal. However, in the new generation of digital cross-connect systems (DCSs) with optical interfaces, the low-speed demands can be processed and a new light path can be created to transmit the remaining demand to the final destination.

Referring now to FIG. 3, an embodiment of a second order single layer grooming model is shown and is generally designated 100. As depicted in FIG. 3, the second order single layer grooming model 100 includes an origination central office (CO) 102 and a destination CO 104. The second order single layer grooming model 100 includes a first hub node 106 and a second hub node 108. Each hub node 106, 108 can include a wide-band digital cross-connect system (WDCS), which can allow cross-connecting of the tributary services at the tributary level and perform traffic grooming.

FIG. 3 shows that the origination CO 102 can be connected to the destination CO 104 via a direct CO-to-CO connection 112, e.g., an STS-1 connection. The origination CO 102 can also be connected to the first hub node 106 via a first spoke connection 114, e.g., an STS-1 connection. In turn, the first hub 106 can be connected to the second hub 108 via a hub-to-hub connection 116, e.g., an STS-1 connection. Also, the second hub 108 can be connected to the destination CO 104 via a second spoke connection, e.g., an STS-1 connection. Finally, FIG. 3 illustrates that the first hub 106 can be connected to the destination CO 104 by a hub-to-CO cross-connection 120, e.g., an STS-1 connection.

It is to be understood that for the second order single layer grooming model 100, grooming DS1/VT1.5 demand traffic increases network utilization efficiency by finding a most economic mix between building direct STS-1 connections between COs or spoke STS-1 connections to transport DS1/VT1.5 traffic to a WDCS located in an intermediate hub node for grooming with other demands to be sent to the same destination. Further, it is to be understood that there are two distinct decision thresholds in the second order single layer grooming model 100 shown in FIG. 3. First, there is the origination CO 102 to destination CO 104 grooming threshold (TH12az). Second, there is the first hub node 106 to destination CO 104 grooming threshold (TH12hz).

The decision to build a direct STS-1 connection between the origination CO 102 and the destination CO 104 in order to transport a particular set of DS1/VT1.5 end-to-end demands is based on a “TH12az” value. In other words, if the demand during a given planning year is more than “TH12az” DS-1s, a direct STS-1 connection is built. On the other hand, the demand lower than the threshold is routed to a WDCS located in a hub node, e.g., the first hub node 106, through an STS-1 spoke connection to be groomed with other demands. Further, if the total demand at the first hub node 106 exceeds a “TH12 hz” threshold value, a hub-to-CO STS-1 connection can be built. Otherwise, the demand at the first hub node 106 is routed to the second hub node 108 via the hub-to-hub connection 116.

This algorithm can be considered a second order algorithm, since it considers two levels of hubbing for each demand. For example, as stated above, if a first round of grooming of DS-1s in the first hub node 106 does not exceed the TH12 hz threshold and a hub-to-CO cross-connection is not justified, the demand is routed to the second hub node 108 using the hub-to-hub connection 116. In one embodiment of the second order single layer grooming model 100, the first spoke 114 carries residual or small amounts of bundled traffic that did not pass the first TH12az threshold test. Moreover, in one embodiment of the second order single layer grooming model, the second spoke 118 can be an STS-1 connection between the second hub node 108 and the destination CO 104 and can transport remaining DS1/VT1.5 traffic without a grooming threshold check. In one embodiment, the second order single layer grooming model 100 can be considered “symmetric” if TH12az=TH12 hz.

FIG. 4 depicts an exemplary embodiment of a second order multi-layer grooming model, generally designated 150. As illustrated in FIG. 4, the second order multi-layer grooming model 150 includes an origination central office (CO) 152 and a destination CO 154. Moreover, the second order multi-layer grooming model 150 includes a first hub node 156 and a second hub node 158. As demonstrated in FIG. 4, each CO 152, 154 can include a SONET ADM 160, or other type of next generation SONET equipment (NGS), and an OADM 162. Further, each hub node 156, 158 can include a WDCS 164 and an MSTS 166 with OXC functionality. It is to be understood that the WDCSs 164 and the MSTSs 166 are capable of multi-layer grooming in both electrical and optical levels. In the present embodiment, the input traffic to the origination CO 152 can include DS-1, DS-3, OC-3, OC-12, OC-48 (2.5 Gbps), OC-192 (10 Gb), 1 Gigabyte Ethernet (1 GigE), 10 GigE, and transparent wavelength services, λ.

It can be appreciated that the second order multi-layer grooming model 150 can include an electrical level that is established by the interconnection of the SONET equipment, e.g., the SONET ADMs 160 and the WDCSs 164. Moreover, the second order multi-layer grooming model 150 can include one or more optical levels established by the interconnection of the optical equipment, e.g., the OADMs 162 and the MSTSs 166. It can also be appreciated that a SONET level is capable of transporting time division multiplexing (TDM) services such as DS-1, DS-3, OC-3 and OC-12. A WDM optical level is capable of carrying OC-192 (10 Gb), 10 GigE, and transparent wavelength services, λ. OC-48, 1 GigE, Enterprise Systems Connectivity (ESCON), Fiber Connectivity (FICON), and Fiber Channel services can be carried over both SONET and WDM optical networks.

As illustrated in FIG. 4, the origination CO 152 can be connected to the destination CO 154 via plural direct CO-to-CO connections 168. The direct CO-to-CO connections 168 can include, e.g., one or more STS-1 connections, one or more OC-3 connections, one or more OC-12 connections, one or more OC-48 (2.5 Gbps) lightpath connections, and/or one or more OC-192 (10 Gb) lightpath connections. The origination CO 152 can also be connected to the first hub node 156 via a first set of spoke connections 170, e.g., one or more STS-1 connections. FIG. 4 further indicates that the first hub node 156 can be connected to the second hub node 158 via plural hub-to-hub connections 172, e.g., one or more STS-1 connections, one or more OC-48 (2.5 Gbps) lightpath connections, and/or one or more OC-192 (10 Gb) lightpath connections. Also, the second hub 158 can be connected to the destination CO 154 via a second set of spoke connections, e.g., one or more STS-1 connections. Finally, FIG. 4 illustrates that the first hub 156 can be connected to the destination CO 154 by plural hub-to-CO cross-connections 176, e.g., one or more STS-1 connections, one or more OC-48 (2.5 Gbps) lightpath connections, and/or one or more OC-192 (10 Gb) lightpath connections.

FIG. 5 illustrates the general operating logic of an embodiment of a grooming tool. Commencing at block 200, network demand data is received at the grooming tool 22 (FIG. 1). In one embodiment, the network demand data can include published network demand data and unpublished network demand data. The published demand data, for example, can include DS-1 end-to-end speculative forecast for non-switched services with 128, 256, 384, 1.5M, 1.5Z, T1, T1ZF, T1ZFN planning groups. Also, the published network demand data can include DS-3, OC-3, OC-12, and OC-48 high capacity end-to-end demand with 45M, EC1, SN32, SN33, STN, OC-3, STS3, OC-12, STS-12, OC-48, and OC-192 planning groups. Moreover, the published network demand data can include general trunk forecast (GTF) switch services forecasts including message trunk groups, competitive local exchange carrier (CLEC) and local number portability (LNP).

Further, a user, e.g., a network planner, can add other required demands not included in the published forecasts, i.e., 1 GigE, 10 GigE and wavelength demands. The unpublished network demand data can also include end-to-end demands representing asynchronous transition plan by route, copper migration plan, dial-for-dial end-to-end demand, and long distance (LD) demand on interoffice (IOF) transport rings. The unpublished network demand data can also include broadband demand, e.g., ATM, frame relay (FR), Internet, video, etc. Additionally, the unpublished network demand data can include customer specific demands, e.g., inquiries on DS-1, DS-3, OC-3, etc.

Continuing the description of the flow chart depicted in FIG. 5, at block 202, all demands are aggregated. Next, at block 204, all demands are groomed based on the above-described second order multi-layer grooming model 150 (FIG. 4). Proceeding to block 206, the optimal paths for carrying traffic are determined. At block 208, an optimal routing plan including the optimal paths determined above is output. The logic then ends at state 210.

Referring now to FIG. 6, a flow chart to illustrate detailed operating logic of an embodiment of a grooming tool is shown and commences at block 250, where one or more direct connections are created between an origination CO and a destination CO. In the present embodiment, the direct connections can carry a mixture of demand types, and the types of direct connections can be based on the “az” thresholds shown in Table 1. For example, if the demand is greater than a TH12az value then a DS-3 connection is built. Table 2 shows exemplary values for the “az” thresholds and “hz” thresholds for a symmetrical model. So, based on Table 2, if the demand is given as DS-1 bundles and the number of DS-1 bundles exceeds, e.g., twenty (20), then a DS-3 connection can be built. Further, if the demand is given as DS-1 bundles and the number of DS-1 bundles exceeds, e.g., sixty (60), then an OC-3 interface is used. In the present embodiment, the sequence in which the connections should be built is as follows: OC-192 (10 Gb), OC-48 (2.5 Gbps), and STS-1. In other words, if the demand traffic calls for an OC-192 (10 Gb) connection, then an OC-192 (10 Gb) lightpath connection should be built

Continuing the description of the flow chart, at block 252, all OC-192 (10 Gb) and OC-48 (2.5 Gbps) bundles are set aside. Proceeding to block 254, the location of a first hub node corresponding to the origination CO is determined. In one embodiment, the location of the first hub node can be based on a user selection and an existing hubbing plan. On the other hand, the direction of each demand can be estimated based on the vertical and horizontal coordinates of the origination CO and destination CO. Then, the shortest hub location path can be assigned to the demand. It can be appreciated that, in general, each demand type can have a different hub location. However, in the present embodiment, it is assumed that the first hub node has the capability and capacity to groom different types of demand in one location.

Moving to block 256, a first set of temporary spoke connections are created between the origination CO and the first hub node. In one embodiment, the temporary spoke connections are STS-1 connections that carry residual DS-1 demand from the origination CO to the first hub node where that demand can be groomed with other residual demand, e.g., from another origination CO. Once groomed, the demand at the first hub node may be large enough to require a direct connection from the first hub node to the destination CO. Otherwise, a temporary hub-to-hub connection can be built from the first hub node to a second hub node and a temporary second spoke connection can be built from the second hub node to the destination CO.

Next, at block 258, the cost of direct STS-1 connections between the origination CO and the destination CO is evaluated for the remaining demands to determine a first cost. At block 260, the cost of required STS-1 spokes between the origination CO and the first hub node is calculated. And, the incremental cost of one or more OC-192 (10 Gb) light path and/or OC-48 (2.5 Gbps) light path to carry this and other remaining demands from the first hub node to the destination CO is added to that cost to get a second cost. In one embodiment, the spare capacities of the first set of temporary spoke connections are used without incurring any cost increase. Continuing to block 262, the first cost is compared to the second cost and the plan with lowest cost is selected. The logic then continues to block 264 of FIG. 7.

At block 264, in the first hub node, all demands destined for the destination CO are gathered. The connections from the first hub node to the destination CO are sized using the “hz” threshold values in Table 1 and the grooming sequence listed above. Next, at block 266, OC-192 (10 Gb) and OC-48 (2.5 Gbps) bundles are set aside to be assigned to lightpaths. Moving to block 268, the cost of direct STS-1 connections between the first hub node and the destination CO are evaluated for the remaining demands to get a third cost. Then, at block 270, the incremental cost of one or more OC-192 (10 Gb) light paths and/or one or more OC-48 (2.5 Gbps) light paths plus the cost of required STS-1 spokes between the second hub node and the destination CO are calculated for the remaining demands to get a fourth cost. In one embodiment, the spare capacities of the second set of temporary spoke connections are used without incurring any cost increase.

Continuing to block 272, the third cost is compared to the fourth cost, and the plan with lowest cost is selected. Thereafter, at block 274, a routing plan based on the lowest costs as determined above is created. At block 276, the routing plan is output. The logic then ends, at state 278.

It can be appreciated that the present method is useful for grooming demand traffic within a multi-level network, e.g., a network having an electrical level and one or more optical levels. Further, the present embodiment can enable the development of efficient transition plans from existing electrical networks to optical networks. Additionally, the method according to the present embodiment can enable network designers to assess and evaluate different hubbing and routing methods and then, select the most efficient one. Also, the present embodiment can enable network designers to provide CO traffic requirements for vendors' equipment selection and configuration evaluation. The method disclosed above can also be used for multi-period planning and network evaluation.

Further, it can be appreciated that by grooming demand traffic using the above-described embodiments, the number of SONET ADMs required by a network can be reduced. As such, the cost of building a network can be reduced. Moreover, the present method can be applied to both ring and mesh topologies having an arbitrary number of nodes under both uniform and non-uniform, i.e., arbitrary, traffic in order to minimize the costs associated with ADMs or OADMs.

The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all modifications, enhancements, and other embodiments, which fall within the scope of the appended claims.

TABLE 1 Threshold Values For Multi-Layer Grooming. From To DS-1/VT1.5 DS-3/STS-1 OC-3 OC-12 OC-48 DS-3/STS-1 TH12az, TH12hz OC-3* TH13az, TH23az, TH13hz TH23hz OC-12* TH14az, TH24az, TH34az, TH14hz TH24hz TH34hz OC-48** TH15az, TH25az, TH35az, TH45az, TH15hz TH25hz TH35hz TH45hz OC-192** TH16az, TH26az, TH36az, TH46az, TH56az, TH16hz TH26hz TH36hz TH46hz TH56hz *Interfaces between ADM/NGS, OADM, WDCS and MSTS **Light path connections

TABLE 2 Exemplary Symmetrical Threshold Values For Multi-Layer Grooming. From To DS-1/VT1.5 DS-3/STS-1 OC-3 OC-12 OC-48 DS-3/STS-1 20 OC-3* 60 2 OC-12* 240 9 3 OC-48 960 34 12 3 (2.5 Gbps)** OC-192 3840 135 45 12 3 (10 Gbps)** *Interfaces between ADM/NGS, OADM, WDCS and MSTS **Light path connections 

What is claimed is:
 1. A method comprising: determining a first estimated cost of forming sets of direct connections between an origination central office and a destination central office, the sets of direct connections including a first connection and a second connection, wherein a speed of the first connection is faster than a speed of the second connection; determining a second estimated cost of forming a spoke connection between the origination central office and a hub node, the spoke connection to carry first residual demand traffic from the origination central office to the hub node, wherein the first residual demand traffic is not carried to the destination central office via the sets of direct connections; selecting a connection between the hub node and the destination central office based at least in part on a comparison of total residual demand traffic to a plurality of threshold values in a threshold matrix, wherein each of the plurality of threshold values corresponds to a network connection type and a connection interface, wherein the total residual demand traffic is based on the first residual demand traffic and the second residual demand traffic; adding to the second estimated cost an incremental cost of forming the connection between the hub node and the destination central office, wherein the connection is configured to carry the first residual demand traffic and second residual demand traffic from the hub node to the destination central office, wherein the second residual demand traffic is received at the hub node from other origination central offices; and determining whether the first estimated cost exceeds the second estimated cost.
 2. The method of claim 1, wherein the connection between the hub node and the destination central office is further selected from a plurality of connection types based on a grooming sequence and wherein the grooming sequence is in an order from highest connection speed to lowest connection speed.
 3. The method of claim 1, wherein, when the total residual demand traffic at the hub node exceeds a particular threshold value of the plurality of threshold values, the connection between the hub node and the destination central office is selected from the network connection type corresponding to the particular threshold value and the connection uses the connection interface corresponding to the particular threshold value.
 4. The method of claim 1, wherein the first residual demand traffic is gathered from traffic assigned for transmission over the second connection.
 5. The method of claim 1, wherein the first residual demand traffic is gathered from traffic assigned for transmission over the first connection, wherein the connection between the hub node and the destination central office is direct.
 6. The method of claim 1, wherein the connection between the hub node and the destination central office is a light path connection.
 7. The method of claim 1, wherein forming the connection between the hub node and the destination central office includes: forming an inter-hub light path connection between the hub node and a second hub node, and forming a second spoke connection between the second hub node and the destination central office; and wherein determining the second estimated cost based on the incremental cost of forming the connection between the hub node and the destination central office includes determining a first incremental cost of forming the inter-hub light path connection and a second incremental cost of forming the second spoke connection.
 8. The method of claim 7, wherein the second spoke connection is an electrical signal connection.
 9. The method of claim 1, wherein the second connection is an electrical signal connection.
 10. The method of claim 1, wherein the spoke connection is an electrical signal connection.
 11. The method of claim 1, wherein the spoke connection is formed when the first residual demand traffic is below a threshold value.
 12. A system comprising: a processor; and a memory accessible to the processor, the memory including instructions that, when executed by the processor, cause the processor to perform operations comprising: determining a first estimated cost of forming sets of direct connections between an origination central office and a destination central office, the sets of direct connections including a first connection and a second connection, wherein a speed of the first connection is faster than a speed of the second connection; determining a second estimated cost of forming a spoke connection between the origination central office and a hub node, the spoke connection to carry first residual demand traffic from the origination central office to the hub node, wherein the first residual demand traffic is not carried to the destination central office via the sets of direct connections; selecting a connection between the hub node and the destination central office based at least in part on a comparison of total residual demand traffic to a plurality of threshold values in a threshold matrix, wherein each of the plurality of threshold values corresponds to a network connection type and a connection interface, wherein the total residual demand traffic is based on the first residual demand traffic and the second residual demand traffic; adding to the second estimated cost an incremental cost of forming the connection between the hub node and the destination central office, wherein the connection is configured to carry the first residual demand traffic and second residual demand traffic from the hub node to the destination central office, wherein the second residual demand traffic is received at the hub node from other origination central offices; and determining whether the first estimated cost exceeds the second estimated cost.
 13. The system of claim 12, wherein the hub node comprises a wideband digital cross connect system.
 14. The system of claim 12, wherein the first connection is an optical signal connection.
 15. The system of claim 12, wherein the origination central office and the destination central office each include an add-drop multiplexer.
 16. The system of claim 12, wherein the second connection is an electrical signal connection.
 17. The system of claim 16, wherein the electrical signal connection comprises a Synchronous Transport Signal connection of a Synchronous Optical Network.
 18. A computer-readable memory device storing instructions that, when executed by a processor, cause the processor to perform operations comprising: determining a first estimated cost of forming sets of direct connections between an origination central office and a destination central office, the sets of direct connections including a first connection and a second connection, wherein a speed of the first connection is faster than a speed of the second connection; determining a second estimated cost of forming a spoke connection between the origination central office and a hub node, the spoke connection to carry first residual demand traffic from the origination central office to the hub node, wherein the first residual demand traffic is not carried to the destination central office via the sets of direct connections; selecting a connection between the hub node and the destination central office based at least in part on a comparison of total residual demand traffic to a plurality of threshold values in a threshold matrix, wherein each of the plurality of threshold values corresponds to a network connection type and a connection interface, wherein the total residual demand traffic is based on the first residual demand traffic and the second residual demand traffic; adding to the second estimated cost an incremental cost of forming the connection between the hub node and the destination central office, wherein the connection is configured to carry the first residual demand traffic and second residual demand traffic from the hub node to the destination central office, wherein the second residual demand traffic is received at the hub node from other origination central offices; and determining whether the first estimated cost exceeds the second estimated cost.
 19. The computer-readable memory device of claim 18, wherein the second connection is an electrical signal connection. 